1. Technical Field
The present invention relates to an image processing device that applies an image process such as printing or scanning to checks or other sheet media, and relates more particularly to a sheet feeding mechanism that is used in such an image processing device.
2. Related Art
A means of preventing feeding plural sheets together (known as multifeeding) is commonly incorporated in sheet feeding mechanisms known from the related art. Japanese Unexamined Patent Appl. Pub. JP-A-2004-269241 teaches a means of preventing such multifeeding. The sheet feeding mechanism described in JP-A-2004-269241 has a transmitting ultrasonic sensor and a receiving ultrasonic sensor disposed on opposite sides of the conveyance path on the upstream and downstream sides of the feed rollers disposed to the conveyance path. To detect sheet multifeeding, output from the receiving ultrasonic sensor is measured when output from the transmitting ultrasonic sensor is stopped, and the sum of this output plus a specific compensation value is set as the threshold value for multifeed detection. Whether multifeeding occurs during sheet conveyance is determined by comparing this threshold value with the output from the receiving ultrasonic sensor.
However, the detection level of the receiving sensor in this configuration for detecting multifeeding depends greatly upon variations in the performance of the receiving sensor and transmitting sensor and installation. A control process for adjusting the sensors and adjusting the threshold value is therefore required for each device in order to accurately set the threshold value for multifeed detection, thus complicating factory adjustment. In addition, if a sensor fails, the failed sensor must be replaced and the sensor adjustment process repeated, and repair therefore becomes a time-consuming process.
More specifically, because the receiving sensor must detect ultrasonic waves at a precise level when an ultrasonic sensor is used for the sensor as described in JP-A-2004-269241, the complexity of the adjustment process increases accordingly.